(a) Field of the Invention
The present invention relates to a display device.
(b) Description of the Related Art
In general, a liquid crystal display includes two display panels having pixel electrodes, a common electrode, and a liquid crystal layer having dielectric anisotropy between the panels. The pixel electrodes are arranged in a matrix and connected to switching devices such as thin film transistors (TFTs) which sequentially apply data voltages to the pixels. The common electrode is disposed over the entire surface of the display panel and supplied with a common voltage. The pixel electrode, the common electrode, and the liquid crystal layer constitute a liquid crystal capacitor. The liquid crystal capacitor together with the switching element is a pixel unit.
The image data voltages vary the strength of the electric field applied to the liquid crystal layer between the two panels thereby controlling the transmittance of light passing through the liquid crystal layer to display images corresponding to the data voltages. To prevent the degradation of the liquid crystal, the polarities of the data voltages with respect to the common voltage are inverted for each frame, pixel row, or pixel.
However, since the response speed of the liquid crystal molecules is low, it takes time for a voltage (hereinafter referred to as a pixel voltage) charged in the liquid crystal capacitor to reach the target voltage. The target voltage is the voltage that effects a desired luminance. The time depends on the difference between the target voltage and the voltage previously charged on the liquid crystal capacitor. Therefore, when the difference between the target voltage and the previously-charged voltage is large, application of only the target voltage will not be enough to cause the pixel voltage to reach the target voltage during the time when the switching element is turned on.
In order to solve the problem, a DCC (dynamic capacitance compensation) scheme has been proposed. The DCC scheme employs the fact that charging speed is proportional to the voltage across the liquid crystal capacitor. The data voltage (actually the difference between the data voltage and the common voltage, usually assumed to be 0V), applied to the pixel is chosen to be higher than the target voltage so as to shorten the time taken for the pixel voltage to reach the target voltage. However, in the DCC scheme, frame memories and driving circuits for performing DCC calculation are needed. Therefore, there are difficult problems in circuit design and increased production cost.
To reduce power consumption in display devices of medium or small size, such as mobile phones, row inversion is performed. However, as the resolution of medium or small size display devices increases, so does power consumption. In particular, when the DCC calculation is performed, power consumption is greatly increased due to the additional calculations and circuitry.
The range of data voltage available for image display using row inversion is small in comparison with dot inversion where the polarities of the data voltages are inverted for each pixel. Therefore, in a VA (vertical alignment) mode liquid crystal display, if the threshold voltage for driving the liquid crystal is high, the available range of the data voltage to represent grayscales for image display is reduced by the amount of the threshold voltage. Therefore, the desired luminance cannot be obtained.